A grating is a key component of the monochromators, and its aberration correcting ability is a decisive factor for the properties of the monochromators. In this paper, a spot diagram method with the genetic algorithm is studied. The genetic algorithm is applied to optimize the merit function for its strong ability to search the extremum of nonlinear functions. A comparison between this method and the damped least squares method is given, and the result shows that the proposed method is much superior to the damped least squares method.